1. Field of the Invention
The present invention relates to an improved wind turbine, of the type which employs doubly fed induction generators (DFIG), and a wind park including the same, which permits the use of lighter weight turbines, with the ability to have greater energy capture, more precise control of asymmetrical phases and enhanced maintenance and support of the grid during fault conditions.
Specifically, the present invention relates to wind turbines configured for operation either singly, in small concentrations, or in wind parks, each turbine being a variable speed turbine using a doubly fed induction generator (DFIG) with grid side inverters, having power provided from both their generator stators and rotors, and preferably with major contributions of reactive power for grid voltage support being provided by park level static reactive power sources. As well, the present invention contemplates the use of blade pitch control, which is used to facilitate staying connected with rotor blades maintained at or near their operational speeds before and during a fault.
2. Background of the Invention
A wind turbine converts kinetic energy from the wind into electrical energy for utility power grids. Wind energy is used to turn wind blades of a turbine rotor for rotating a rotor of an electrical generator, with the electricity being supplied to a utility grid.
The electrical power available from a wind driven generator and supplied to a utility grid is a function of the power available from the wind, its speed, losses in the grid and the characteristics of the distribution system and loads. Because wind speed fluctuates, the force applied to the rotor can vary. Power grids, however, require electrical power at a substantially constant voltage and frequency. Most electric power transmission components have a significant reactive component. Therefore voltages in the grid are also a function of the reactive characteristics of loads and components connected to the grid and to prevent damage to equipment, grid voltage must be held within certain tolerances.
A doubly-fed induction generator (DFIG) can supply real power and contribute to voltage control. How that is achieved depends upon whether the level of rotor current provided is greater or less than that needed to provide sufficient flux to generate rated output voltage. When excess current is applied to the rotor, the generator is considered to be overexcited. In this state more flux than necessary is generated by rotor current, the generator supplies or generates reactive power from the stator and, with regard to reactive power, acts like a capacitor. By convention, this type of reactive power is considered to be positive (flowing from the generator) and is typically labeled “+Q”.
If the generator receives too little rotor current, it is considered to be underexcited. In this state it absorbs reactive power into the stator to help supply flux not provided by its rotor. By convention, reactive power absorbed into the stator is considered to be negative (flowing into the generator) and is typically labeled “−Q”.
A generator's rating, and hence its frame size, is determined by its ability to deliver real and reactive power, and a key consideration relates to the potential adverse effect of heat generation. The ability to deliver real and positive reactive power (+Q) is dependent on rotor current. Rotor heating is directly proportional to the square of total rotor current (I2R) and is therefore proportional to the vector sum of direct and quadrature components of the rotor current. The direct component of rotor current is responsible for generating flux while the quadrature component is responsible for producing torque and power. Rotor heating is also due to rotor core losses from excitation flux. Any increases in rotor current to increase reactive power generation therefore increases rotor heating. In doubly fed induction generators, there are therefore power output limits mandated by heating considerations.
Although absorption of reactive power does not increase rotor heating above what would be experienced by delivery of only real power, the increase in flux due to absorption of reactive power causes excess heating of end sections of a generator's stator. Therefore, there are also power output limits of DFIG systems due to stator end heating.
According to one aspect of the present invention, it is preferred that real power production is provided by the wind turbine generators and reactive power production/absorption is provided by park level Flexible Alternating Current Transmission Systems (“FACTS”) and Static Synchronous Compensator (“STATCOM”) like devices. As a consequence, lighter weight generators can be provided, while maintaining benefits unique to DFIG systems.
Among the benefits of a DFIG system, is its ability to capture energy over a wide speed range by use of rotor current control. Below synchronous speed, power is fed to the generator rotor at a frequency that is the difference between the rotational speed of the rotor and that of equivalent mechanical speed of the grid, allowing the DFIG to provide fixed frequency power from its stator. Above synchronous speed, power is withdrawn from the generator rotor at the appropriate frequency, again allowing the DFIG to provide fixed frequency power from its stator. Above synchronous speed, the power available from the rotor has often been directed into dissipative elements in prior art systems.
More recently, wind power has become a larger contributor of power to an electric grid and use of the power from the rotor as a resource for the grid, rather than having it dissipated, has become more desirable, particularly if the power is delivered to the grid via a DC link and a grid side inverter. Thus, power flow may be maintained even in the short interval when a rotor may not be controlling stator current, as may occur when a sudden drop of grid voltage causes demagnetization of a DFIG. Providing support to the grid through a grid inverter has the added benefit of reducing the load on dissipative elements in the rotor or DC link circuit that is created by the effects of demagnetization.
The present invention, through its use of a DC Link and active grid inverter in lieu of a passive rectifier not only provides support to the grid, but also provides a reduction in grid harmonics.
Moreover, the present invention, in its preferred system includes scalar rather than field oriented control (FOC) of rotor excitation. Control of rotor excitation can be accomplished in at least two ways, namely field oriented control or scalar control. FOC involves transforming AC signals representing three-phase generator stator output quantities in a fixed reference frame into parameters that are essentially fixed in a reference frame that rotates with a rotor flux vector and allows use of DC values. However, with FOC, otherwise useful information regarding the AC current in each phase is lost in the transformation process. FOC presupposes that the three phase AC currents are equal and sum to zero. Because in certain instances the AC signals are asymmetrical (that is, not equal), useful AC information may be lost during the transformation of AC signals from the stationary frame into a rotating frame.
As a consequence, FOC is unable to be used in a system that independently controls the electrical quantities (e.g., voltage, current) of each phase of the power grid. Theoretically, each phase of an ideal power grid should not vary. However, the electrical quantities on each phase of a wind turbine generator may vary due to transients on the grid which may cause uneven thermal stress, unbalanced mechanical forces and high DC link voltages. Accordingly, in a system in which control of rotor excitation is to be employed for grid control, it is desirable to correct for the asymmetry for at least the outputs of each of the three phase connections of a DFIG stator.
The present invention therefore incorporates as its preferred embodiment the use of scalar control for both the stator and grid inverter for the same reason: to provide independent phase parameter control. Two alternative scalar control configurations are described that provide for the desired individual phase control. These configurations are based on the use of conventional Proportional-Integral (“PI”) controllers or Resonance PI controllers. Resonance PI controllers have the advantage of offering steady state error control similar to that achievable with conventional PI controllers in DC based controls but have superior performance compared to conventional PI controllers in AC based scalar controls. Both conventional PI controllers and Resonance PI controllers are suitable for use in scalar controllers however.
A Resonance PI controller typically includes at least one resonance term and has a substantially zero phase shift in the vicinity of the resonance. A main advantage of a Resonance PI controller is that it is well suited for tracking a sinusoidal reference or error signal to reduce steady state errors to substantially zero. A Resonance PI controller operates directly on an AC signal and needs no coordinate transformations to achieve the high steady state accuracies typical of conventional PI Controllers operating on DC signals. The Resonance PI controller is therefore suitable for scalar control of a variable speed doubly fed induction generator where rotor frequencies vary with slip and in a grid inverter where the AC signal is fixed at a grid frequency. One form of a PI Resonance Controller in Laplace transform notation is given as:
            H      RC        ⁡          (      S      )        =            K      p        +                  K        i            ⁢                        2          ⁢                                          ⁢                      ω            d                    ⁢          s                                      S            2                    +                      2            ⁢                                                  ⁢                          ω              d                        ⁢            S                    +                      ω            n            2                              
In the equation above, parameters Kp and Ki are proportional gain and integral gain respectively, ωn is the resonant frequency and ωd is a dampening operating parameter used to describe the sharpness of the characteristic near the resonant frequency. For rotor current control, ωn is a slip frequency that is variable and therefore tracked in a preferred embodiment while when used for control of the grid inverter ωn is fixed at the grid frequency.
The DFIG system of the present invention is specifically adapted to stay connected to the grid during a fault and provide support to the grid. The grid support related to counteracting the effect of a fault is generally an effort to accomplish two objectives. The first is to help clear the fault and with the assistance of reactive power sources raise system voltages during the fault. The second is to minimize the amount of time required to place a wind turbine back on line generating power after the fault condition has been addressed.
Although it seems counterintuitive, it is advantageous for affected wind turbines to continue to provide output current at substantially the same magnitude as that which was present prior to the fault and not reduce it. A normative output current is better able to actuate the protective devices (such as circuit breakers) and therefore potentially shorten the time to isolate a fault.
The grid support required often varies, depending upon the requirements of the grid as set forth in Grid Codes. Grid Codes around the world require different behavior during a low voltage grid fault. Some require full reactive current and as much active current as possible during the grid disturbance. Others prioritize the active current. Although it would be desirable to be able to maximize real and reactive current at the same time, component heating, whether of the rotor of a DFIG or the current carrying elements of a partial or full converter, is a function of both the real and reactive components of the current being carried. Therefore if it is desired to maximize real current, then the reactive current component must be minimized. Likewise if reactive current were to be maximized, then the real current component of the total must be minimized.
The present invention in one of its preferred forms has among its advantages, the ability to maximize both the reactive and real current components available from a wind turbine or grouping of wind turbines, by providing a system which permits the use of a separate reactive power supply to handle reactive current requirements during a fault with the wind turbines therefore able to maximize real power. As a consequence, the wind turbine generators can be made smaller, with an attendant savings in size, weight and cost.
Staying connected during fault conditions can have adverse consequences to mechanical components as well. If power load on a wind turbine is reduced, as it is in a low voltage fault condition, the blades of the turbine could accelerate and damage may occur. Nevertheless, there is a significant benefit in permitting the wind turbine rotor to be kept running at or near the pre-fault speed, so the generator can be more rapidly be operational when the fault is cleared.
To this end, according to another aspect of the present invention, rotor speed control is maintained by setting the blade pitch angle to a value that balances power load presented to the rotor by mechanical and electrical loads during the fault.
A variable speed wind turbine extracts the maximum it can from the wind when the blade tip speed to wind speed ratio is a constant at or near the particular design value for the particular wind turbine design. Stated differently, as the wind speed increases, the rotational speed of the generator increases and brings it closer to the upper speed limit of the rotor and generator, and therefore it is not possible to operate a wind turbine at its optimum blade tip-wind speed ratio over the whole wind speed range.
In the mid- to upper-speed range, which is the range at which the wind turbine is most efficient, the blade tip to wind speed ratio is held constant by balancing the power output from the system to that available from the wind. That is, the power commanded by the wind turbine is derived from knowledge of the wind speed and is set to that value.
At higher wind speeds the wind turbine operates at its optimum blade tip to wind speed ratio (typically in the range of about 6 to 10) because to keep the ratio constant would require a rotational speed for the generator that would exceed its limits. Therefore when wind speed increases to its nominal speed, and further increases could drive the generator into an unsafe speed range, a generator speed reference is clamped to the nominal speed point. If wind speed increases further, the commanded power output is limited to a fixed value and blade pitch is varied to keep the power taken from the wind equal to the power necessary to keep the wind turbine rotor, and hence maintain the generator at its nominal speed.
In the case of higher wind speed (as in the case of the lower speed range as well), there must be a balance between power captured by a rotor system and the power outputted from the turbine plus losses in the various wind turbine systems. However, a difference between operation in these ranges is that in the optimum speed range, the blades are set to extract as much power as available, while in the higher speed range the blades' pitches are set to take just enough power to meet the value commanded for full power which is less than what is actually available in the wind.
When either balance is disturbed, as in the case of a sudden low voltage fault or power error, the known practice is to use a rapid blade pitch change to a no acceleration pitch angle (NOA) followed by shutdown to prevent equipment damage due to over currents and overspeed conditions. According to another aspect of the present invention, a rapid change of the pitch angle to the NOA angle to prevent overspeed is enhanced by the added capability of pitch control during the fault.